There is conventionally disclosed an obstruction detection device for detecting a distance between an object and a sensor by measuring a time required for the return of a reflected wave of an ultrasonic wave transmitted from the sensor, and acquiring a two-dimensional position of the object by delay-summing the signals from a plurality of ultrasonic sensors. For example, in Patent Document 1, there is disclosed a mobile machine including an ultrasonic sensor for receiving the reflections of an ultrasonic wave by a plurality of sensors, and delay-summing the resultant thereof, thereby creating a two-dimensional distance image in a specific horizontal plane.
However, a conventional obstruction detection method has the following problem: when a two-dimensional distance image in a specific horizontal plane is created, a noise or a virtual image appears. As a method for solving this problem, for example, Patent Document 2 discloses the following configuration: the number of ultrasonic sensors to be disposed are increased, and disposed in a close array to thereby reduce the noise, and also the information in the vertical direction of an object is obtained.